I am pretty sure that the people saying it's easier in 5e than 1e haven't actually looked at the stats and are just going off of some '1e = hard, 5e = easy' idea. In 1e a purple worm had AC6 (equivalent to scale mail, or AC14 in 5e terms), average of 75 HP and a 9" move. In 5e, it has AC 18 (full plate, or AC 2 in 1e terms), average of 225HP, and a 50' move. That's more than double the movement (trashing escape strategies), more than triple the raw HP, and an even larger increase in effective HP (since the AC went up by 5 and more 5e spells use attack rolls). It's even worse since, looking back at the modules, the random encounter in the module says to use 54 HP while the 5e conversion just says to use standard.
Sure, PC HP increased; a magic user with no con bonus would have an average of 14 HP, while today he'd have 20, and con bonus arguably would be more likely, bumping it to 30 with +2. But that's the worst case - a fighter would average 32 then and 47 today (Assuming 0 con then and +3 today) while a barbarian woud go from 38 to 53 (assuming 0/+3 like before). But damage went up even more - the bite damage close to doubled (2d12 to 3d8+9), the damage from the swallow doubled from 3d6 to 6d6, and the rule limiting the stinger went away adding second, stronger attack with a DC 19 poison for 42 damage. The standard damage from bite + swallow is 43, while the stinger is 63 - so the swallow is almost enough on average to take down the fighter or barbarian, who will then fall on the second (or third if raging) round, while the stinger takes down even a raging barbarian on average. The resilience of tough characters to attacks if we assume no con bonus in 1e is about actually about the same (13+10 on swallow, then 10 more kills the fighter, then 10 more kills the barb), but there are two of them in the 5e fight and fewer players.
Party damage sees the biggest increase, but I don't think that it went up the 4x needed to compensate for 1e having 1.5 times party size and the 5e worm having 3x the HP (to say nothing of the 5x hp for the actual case). (In comparing damage, note that the party needs to do enough to kill the worm, you can't just look at what one character can do in one round once per rest, especially if that requires a hit roll or failed save, and you have to account for the worm taking people completely out of the fight fast).
Overall, if the 1e version jumps you in melee range (which is what the table entry implies), it's much easier to hurt than the 5e version, at best can take down 1 character per round, and you can run away from it on foot. In 5e, it's faster than any normal character on foot, and just as fast as their camel mounts if they manage to stay riding. I really don't see how you can actually compare the stats and claim that the 1e version is a harder fight, especially with the huge speed increase.
Eh, wouldn't that be 90ft in 1e? Seem to recall 1"= 10ft.
No; if it was a distance 9" would be 90 feet or 90 yards depending on indoors or outdoors, but as a movement works differently (largely IMO because rounds were longer). A regular human had a move of 12", 9" was your move heavy armor or with moderate encumbrance, and I think 6" for heavy. A 9" move would correspond to either a 20' or 22.5' move in 5e (depending on whether you do a functional comparison or literal ratio). I don't think the people talking about running away actually noted that the speed of the monster relative to PCs more than doubled from 1e to 5e and took it into account for the difficulty of running. If the characters start to fight it, then realize they're outclassed in 1e, they can easily get away since even the slow people move just as fast as the worm. In 5e, walking PCs are slower than it and even PCs on camels can't actually outrun it.
The desert in question counts as "a very spacious area" and a party of adventurers, even 3, unless they all cluster together, could be subject to the sting.
If 3 people counts as 'numerous' there's no point in having the qualifier, so I don't accept that interpretation. If you want to run the 1e monster with the ability set it had in 2e (where the tail restriction was removed) that's fine to do in your game, but what I'm comparing here is the RAW 1e version and the 5e version, not the 2e to 5e or your house rules to 5e.
While the Purple Worm wasn't the Tarrasque, it was one of the signature tough hombres in 1e. As far as this module goes in 1e, if you saw one, you would run; my confusion at your post is that the 5e version is *easier* IMO than the 1e because of the whole Big Bad problem that has been discussed here before.
I don't think your claim that the 5e version is easier than the 1e version is based off of the actual stats, as I pointed out above. The 'single Big Bad' problem doesn't happen if the single big bad is one-shotting players, can shrug off the player's best attacks and is immune to a lot of the 'screw the single target' abilities they can bring to bear at that level.
This. Also, I think the OP is forgetting how 1e PCs were. A 10th level Magic User only had an average of 25 hit points. A 10th level fighter might have 60-70 hp. The adventure is for levels 5-7. A 6th level MU will have 15hp, a 6th level fighter will have 40ish. So a 1e purple worm could one shot kill many PCs, not even counting the poison or swallow ability.
I think you're the one forgetting how HPs were (and I have no idea why you're using 6th level HP instead of 5th) I pointed out the HP comparison above, getting double the hit points in the best case (Magic User and assuming low 1e stats) or about 1.5 times in the case most likely to soak hits (fighter or barbarian, again assuming low 1e stats) doesn't make up for quadroupling the damage output AND having a smaller party. And you're forgetting how fragile and slow 1e purple worms were compared to the modern version. 75 HP, AC6 (14 now), slower move than a regular human vs 250hp, AC 18 (2 then), faster move than a regular human as as fast as their horses.
And again the poison is irrelevant because it's not usable in the fight by the version of the monster we're discussing.